The present invention relates to a spin excitation method and apparatus and a magnetic resonance imaging apparatus, and more particularly to a spin excitation method and apparatus which excite spins within an object to be imaged by a pulse sequence containing RF (radio frequency) pulses, and a magnetic resonance imaging apparatus employing such a spin excitation apparatus.
When spins within an object to be imaged are RF-excited in a magnetic resonance imaging apparatus, the apparatus suppresses the SAR (specific absorption rate) by the RF excitation below a predetermined standard value to ensure safety of the object to be imaged. For this purpose, each time scan conditions are specified, the magnetic resonance imaging apparatus predicts the SAR in performing imaging under the conditions, and if the predicted SAR exceeds the standard value, the scan conditions are modified. The modification of the scan conditions is conducted automatically or by prompting an operator to re-specify the scan conditions.
Since the SAR is approximately proportional to the square of the frequency of the RF excitation signals, if the magnetic resonance imaging apparatus employs a high-strength magnetic field on, for example, the order of 3 T (Tesla) as a static magnetic field, the SAR tends to deviate from the limit due to an increase of frequency of the RF excitation signals, thereby increasing the occurrence of the scan condition modification. While the scan condition modification for reducing the SAR is usually carried out by reducing the number of slices scanned during 1 TR in multislice scanning, reducing the number of slices lowers the imaging efficiency.
It is an object of the present invention to provide a spin excitation method and apparatus for performing imaging with good efficiency while keeping within an SAR limit, and a magnetic resonance imaging apparatus employing such a spin excitation apparatus.
In accordance with a first aspect of the invention, there is provided a spin excitation method for exciting spins within an object to be imaged by a pulse sequence containing RF pulses, the method comprising the steps of predicting an SAR of the object to be imaged in executing the pulse sequence; and adjusting at least one among the number of pulses, pulse waveform and pulse width of the RF pulses in the pulse sequence so that the predicted SAR value falls within a predetermined limit.
In accordance with a second aspect of the invention, there is provided a spin excitation apparatus for exciting spins within an object to be imaged by a pulse sequence containing RF pulses, the apparatus comprising: SAR prediction means for predicting an SAR of the object to be imaged in executing the pulse sequence; and RF pulse adjusting means for adjusting at least one among the number of pulses, pulse waveform and pulse width of the RF pulses in the pulse sequence so that the predicted SAR value falls within a predetermined limit.
According to the present invention, at least one among the number of pulses, pulse waveform and pulse width of RF pulses in a pulse sequence is adjusted so that a predicted SAR value falls within a predetermined limit. Since the number of slices during 1 TR is therefore not reduced, lowering of imaging efficiency is prevented.
Therefore, the present invention can provide a spin excitation method and apparatus for performing imaging with good efficiency while keeping within an SAR limit and a magnetic resonance imaging apparatus employing such a spin excitation apparatus.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.